Under circumstances where suspended particulate matter must be removed from a gas stream, it has long been the practice to use a filter assembly containing a replaceable paper or other suitable filter element. In instruments which measure the composition of automobile exhaust gases, for example, it is customary to filter suspended soot particles from the exhaust gases before the latter are applied to the sensitive interior of the instrument. One filter assembly of this type, which may be termed an axial-flow filter, includes a filter element mounted within an elongated or tubular housing. In filter assemblies of this type, the sample gas is introduced at one end through an inlet that lies generally parallel to the axis of the housing, passes through the filter element, and exits at the opposite end through an outlet that also lies generally parallel to the axis of the housing.
Another type of filter assembly, commonly known as a pancake filter, includes a filter element that is mounted in a flattened or disc shaped housing. In filters of this type, the inlet and outlet of the filter are frequently not aligned with the axis of the housing. On the contrary, the inlet and outlet may penetrate the same end of the housing or may even penetrate the housing from the side. Filter assemblies of this type are often used in applications in which the space available for the filter assembly, or the directions from which the inlets and outlets can enter the housing, are limited by the design of the surrounding devices.
One problem with existing pancake filters is that they establish internal gas flow patterns such that suspended particulates become concentrated on the portion of the filter element that is closest to the outlet of the filter. This uneven accumulation of particulates tends to reduce the useful life of the filter element below that which would be the case if particulates accumulated evenly over the surface thereof. This causes it to be necessary to replace filter elements relatively frequently, resulting in inefficient and uneconomical filter element usage. More importantly, however, the relatively frequent changing of filter elements requires relatively frequent interruptions in the taking of measurements on the instruments with which the filter assembly is used. Alternatively, the failure to change filter elements relatively frequently can cause drift in an instruments' output readings over a period of time.
In view of the foregoing, it will be seen that, prior to the present invention, it was often necessary for an instrument designer to choose between using a bulky filter assembly with desirable particulate deposition characteristics, and using a compact filter assembly with undesirable particulate deposition characteristics.